In SolidWorks 2009 SpeedPak was introduced to help with performance problems in large assemblies.
Here's an excerpt on SpeedPak from the Help files:
SpeedPak creates a simplified configuration of an assembly without losing references. If you work with very large and complex assemblies, using a SpeedPak configuration can significantly improve performance while working in the assembly and its drawing.
A SpeedPak configuration is essentially a subset of the parts and faces of an assembly. Unlike regular configurations, where you can simplify an assembly only by suppressing components, SpeedPak simplifies without suppressing. Therefore, you can substitute a SpeedPak configuration for the full assembly in higher level assemblies without losing references. Because only a subset of the parts and faces is used, memory usage is reduced, which can increase performance of many operations.
After talking to a number of customers we've found that many of you are struggling with large assembly performance but aren't using SpeedPak to simplify subassemblies.
Why not? I've heard a number of reasons from customers that I've visited but if you could post your answers here for discussion that would be great. And I'll try to answer any questions you may have about the current functionality.